The presence of electrically conductive surfaces on polymeric articles are required in many applications. For example, electrically conductive surfaces are oftentimes required for spray painting articles such as, but not limited to, vehicular body parts such as automobile body fenders, mirror housings, grills, and bumper fascias, as well as aircraft components and household products, such as refrigerators and other like appliances.
Electrically conductive surfaces may also be important in the production of tires. It is known that electrostatic charges are produced by the mechanical and electrical components operating within a motor vehicle. For example, a rotating shaft within a bearing can result in an electrostatic charge being generated within the vehicle. To dissipated this and other charges, electrically conductive rubber tires are employed on these vehicles. Were such dissipation not to occur, interference with electronic components within the motor vehicle would likely occur. Furthermore, static shock might be experienced by the vehicle's passengers as they board the vehicle. High electric charges have also led to safety hazards upon refueling of the vehicle. Accordingly, rubber tires have typically acted as an adequate conduit for the dissipation of such static charges produced from the mechanical or electrical components of a motor vehicle.
Presently, in order to provide electrically conductive surfaces to articles for various applications such as spray painting, the articles are manufactured entirely from conductive compounds of homopolymers or copolymers or blends thereof. The conductivity is achieved by the incorporation of conductive fillers, such as carbon fibers, carbon black particles, metal particles, etc., into the polymer from which the entire article will be formed using preferably conventional molding techniques. Unfortunately, many of these conductive compounds or polymers are very expensive. Therefore, the manufacturing of the article is very expensive.
Articles and products requiring electrically conductive surfaces are usually made by extrusion, compression or injection molding processes. However, some of these processes deleteriously affect the surface conductivity of the article or product.
For example, in the manufacture of articles via the injection molding process, a commonly utilized process for manufacturing these articles, high shear conditions oftentimes occur. These conditions, where the shear rate is typically between about 1000 and 10,000 l/sec., often lead to the migration and separation of the conductive particles, e.g., carbon black or metal particles, from the product surface. That is, when injection molding, the conductive particles within the polymer matrix used to form the article will often tend to be pushed or migrate away from the surface of the article. Thus, the surface of the article will tend to be rich in the polymer matrix and lean with respect to the conductive particles, while the core of the article will tend to be rich in the conductive particles and lean with respect to the polymer matrix.
Another problem with using conductive polymers throughout the entire molded product, besides the expensive cost of the polymers and problem of migration and segregation caused by high shear rate conditions, is that the conductive particles may reduce certain mechanical properties, such as tensile strength, in the article being manufactured. In some cases, it is desirable to maintain the high mechanical properties associated with certain homopolymers or blends thereof.
In tires, the problems with electrical conductivity has arisen due to the fact that tire manufacturers have begun to use less conductive materials, e.g., silica, within the tires which has, in turn, resulted in a decrease in the dissipation of static charge through such tires. Accordingly it is desirable to maintain the excellent advantages provided by the use of silica-filled tires, but yet adequately dissipate the static electricity which builds up in the motor vehicle.
Several patents have taught tires or tire treads having a thin conductive layer on the outermost portion of the tread. However, these tires processes add the conductive layer by spray coating or by production of an additional layer which does not include the advantageous filler. Thus, the filler is never a part of the conductive layer.
Thus, the need exists for a multi-layered article having at least one electrically conductive layer comprising a surface of the article and at least one non-conductive layer adjacent to the electrically conductive surface layer and forming the core, or at least another portion, of the article, both the conductive and non-conductive layers being made from the same or similar base polymer(s) and/or filler(s). In addition to the conductive material in the layer, the conductive layer can be filled with one or more non-conductive fillers, while the non-conductive layer can be filled with non-conductive filler(s). It is also desired to continue using well known processing techniques in the manufacture of these articles.
It will also be appreciated that production of articles having only the surface as a conductive layer may further be useful in solving problems associated with EMI shielding with conductive cores.